Comparing Holy Scripture with other writings, we observe that no book is apparently so full of contradictions as the Bible, and that, not only in minor points, but in the principal matter, in the doctrine how we may come to God and be saved. In one place the Bible offers forgiveness to all sinners; in another place forgiveness of sins is withheld from all sinners. In one passage a free offer of life everlasting is made to all men; in another, men are directed to do something themselves towards being saved. This riddle is solved when we reflect that there are in the Scriptures two entirely different doctrines, the doctrine of the Law and the doctrine of the Gospel!
CFW Walther, The Proper Distinction Between Law and Gospel (first lecture)